The disclosure of Japanese Patent Application No. 2001-263429 filed on Aug. 31, 2001, including the specification, drawings and abstract is incorporated herein by reference in its entirety.
1. Field of Invention
The invention relates to an electric power steering device that corrects, via a motor, a torque variation in a steering link in an automobile, as well as to a control method thereof.
2. Description of Related Art
An electric power steering device that generates a steering assist force using a motor, such as that disclosed in Japanese Patent Application Laid-Open Publication No. 9-254804, has conventionally been known. In this electric power steering device, a first shaft, which is connected to a steering wheel, is connected to a second shaft by a first universal joint, and the second shaft is connected to a third shaft by a second universal joint, such that a steering link having a plurality of steering shafts is constructed. A torque sensor and a steering angle sensor are then provided on the first shaft and a motor that applies a steering assist force to a link that transmits rotational torque from the steering link to the wheels is provided on that link.
This electric power steering device is also provided with a control unit that includes a control circuit, memory, and a drive circuit. In the memory is stored mapped torque correction data in order to obtain a rotational torque variation of the third shaft with respect to the steering force input to the first shaft. The control circuit determines an assist torque value based on a torque detection signal from the torque sensor, an angle detection signal from the steering angle sensor, and the torque correction data from the memory. The drive circuit then drives the motor based on that assist torque value. It is in this way that the steering assist force is obtained.
With the foregoing electric power steering device, however, because the torque is corrected using mapped torque correction data, new torque correction data must be created every time the configuration is changed, e.g., every time the length of the steering shafts or the angle between them and the like are changed, that corresponds to that change, which is troublesome. Moreover, creating that torque correction data based on experimental values increases the number of man-hours increases even more.
Still further, with an electric power steering device that is provided with a variable-ratio steering mechanism that changes the phase between the steering wheel and the end of the steering link opposite the steering wheel, it is extremely difficult to create appropriate correction data, and what is more, the mounting positions of the torque sensor and the steering angle sensor are limited.
In view of the foregoing problems, it is an object of this invention to provide an electric power steering device i) in which torque correction is possible by a simple method, ii) in which a torque sensor, a steering sensor, and a motor, and the like can be mounted to any portion of one of the steering shafts, and iii) which can also be provided with a variable-ratio steering mechanism.
In order to achieve the foregoing objects, an electric power steering device is provided that includes i) a steering link which includes a plurality of connected steering shafts that connect the steering wheel with the steered wheels, ii) a motor that provides a steering torque to assist the steering link, iii) a torque sensor which is provided on one of the steering shafts and which detects a steering torque of the steering shaft, iv) a steering angle sensor which is provided on one of the steering shafts and which detects a steering angle of the steering shaft, v) a calculating unit that calculates a steering assist amount based on the steering torque detected by the torque sensor, the steering angle detected by the steering angle sensor, and predetermined crossed axes angles between the connected steering shafts, and vi) a motor controller that drives the motor so as to apply a steering assist force to the steering link.
In addition, a control method is provided for an electric power steering device including a steering link that includes a plurality of connected steering shafts that connect the steering wheel with the steered wheels, a motor that provides a steering torque to assist the steering link, a torque sensor which is provided on one of the steering shafts and which detects a steering torque of that steering shaft, and a steering angle sensor which is provided on one of the steering shafts and which detects a steering angle of that steering shaft, includes the steps of calculating a steering assist amount based on the steering torque detected by the torque sensor, the steering angle detected by the steering angle sensor, and predetermined crossed axes angles between the connected steering shafts, and driving the motor so as to apply a steering assist force to the steering link based on that steering assist amount.
According to an electric power steering device having the above described configuration and the above-described control method thereof, because the steering assist amount is obtained by calculation from the steering torque, the steering angle, and the crossed axes angle between each of the steering shafts, an appropriate assist force can always be applied to the steering link irrespective of the positional relationship, e.g., the crossed axes angle, between each of the steering shafts, thus giving the driver a uniformly stable steering feel. In addition, because the steering assist amount is calculated based on the steering torque that is detected by the torque sensor, the steering angle that is detected by the steering angle sensor, and the crossed axes angle between the steering shafts, the mounting locations of the torque sensor and the steering angle sensor are able be anywhere on the steering shafts, which improves the degree of freedom in the design of the construction.